What Is Forward Error Correction?
Forward error correction is also called forward error correction (FEC), which is a method to increase the reliability of data communication. In a one-way communication channel, once an error is found, its receiver will not have the right to request transmission again. FEC is a method of transmitting redundant information using data. When an error occurs during transmission, the receiver will be allowed to reconstruct the data.
- Forward error correction is an error control method. It means that the signal is encoded according to a certain algorithm before being sent to the transmission channel. Redundant codes with the characteristics of the signal are added. Technology to decode the signal to find out the error code generated during transmission and correct it [1]
- Compared with digital programs and analog programs, the effect is clearer, the colors are more pure, the permeability is higher, and the picture is free from interference. This is all due to the excellent anti-interference ability of digital signals. In digital signals, in order to prevent external signal interference and protect the signal from mutating, multiple
- Digital signals actually transmit data streams including the following three types:
- Forward error correction ES stream: Also called elementary code stream, it contains a continuous code stream of video, audio or data.
- Forward error correction PES stream: Also called packed elementary stream, the ES stream of elementary stream is divided into different lengths as required
- Take the flow of a product to compare the differences between the three types of data flow:
- If the ES stream is the raw material of the product, then the PES stream is a product just produced by the factory, and the TS stream is the product that is packaged and delivered to the store counter or the user. If the weight of the ES stream is referred to as the net weight, then the weight of the TS stream is called gross weight. What does this metaphor have to do with FEC?
- From PES stream to TS stream, FEC has been added in this process
- The use of FEC can effectively improve the performance of the system. According to Shannon's theorem, the limit performance (Shannon limit) of error-free transmission of noise channels can be obtained, as shown in Figure 2. It can be seen from Figure 2 that the performance of the FEC scheme is mainly determined by three main factors: encoding overhead, decision method, and codeword scheme.
- (1) Coding overhead: The ratio of check bit length (nk) to information bit length k is called coding overhead. The larger the overhead, the higher the theoretical limit performance of the FEC scheme, but the increase is not linear. The larger the overhead, the smaller the performance improvement caused by the increased overhead. The choice of cost needs to be determined according to the requirements of the specific system design.
- Figure 2: Shannon limits for hard-decision FEC and soft-decision FEC
- The research on the application of FEC in optical fiber communication started relatively late. Since Grover first used FEC in optical fiber communication in 1988, FEC applications in optical fiber communication can be divided into three generations.
- The first generation of FEC: Classic hard decision code words, such as Hamming code, BCH code, RS code, etc. The most typical representative codeword is RS (255,239), with an overhead of 6.69%. When the input BER is 1.4E-4, the output BER is 1E-13, and the net coding gain is 5.8dB. RS (255, 239) has been recommended as the ITU-T G.709 standard for long-range and long-range communication systems, which can well match the STM16 frame format and has been widely used. In 1996, RS (255, 239) was successfully used in trans-Pacific and Atlantic ocean communication systems with a length of 7000 km, and the data rate reached 5 Gbit / s.
- The second generation FEC: based on the classic hard decision codewords, the cascade method is adopted, and the interleaving, iterative, and convolutional technical methods are introduced, which greatly improves the gain performance of the FEC scheme and can support 10G and even 40G systems. Transmission requirements, the performance of many programs have reached more than 8dB. The FEC scheme recommended in ITU-T G.975.1 can be used as a representative of the second generation FEC.
- Existing 10G systems mostly use second-generation hard-decision FEC, and hard-decision FEC with larger overhead can support the smooth upgrade of existing systems. For example, a 10G submarine cable transmission system adopts a hard decision FEC scheme with an overhead of 6.69% recommended by ITU-T G.975.1. If a high-performance hard decision FEC with a 20% overhead is used, it can increase the coding gain by about 1.5dB compared to the existing scheme , Greatly improving the performance of the system.
- The third generation of FEC: The application of coherent reception technology in optical communications makes the application of soft decision FEC possible. With larger overhead (20% or more) soft decision FEC schemes, such as Turbo codes, LDPC codes and TPC codes, can obtain coding gains greater than 10dB, effectively supporting long-distance transmission requirements of 40G, 100G to 400G [3] .